Towards the end of the season when my incubator has a bunch of clutches in it, I need to drop the t-stat from 89.5F to 88F to maintain 88F- 88.5F in the tubs. The temps always rise when you actually have some eggs in the tubs. At least this has been my experience the last 2 seasons.
Right now i am sitting at 89.5F on my T-stat and 87.5F in tubs loaded with vermiculite and water. I expect when I start adding eggs the temps will be just where I want them around 88F- 88.5F.
Yeah I have never had to adjust my stats on either of the bators I built.